U.S. Pat. No. 4,510,489, issued to Anderson et al., discloses a magnetomechanical EAS system in which markers incorporating a magnetostrictive active element are secured to articles to be protected from theft. The active elements are formed of a soft magnetic material, and the markers also include a control element (also referred to as a "bias element") which is magnetized to a pre-determined degree so as to provide a bias field which causes the active element to be mechanically resonant at a pre-determined frequency. The markers are detected by means of an interrogation signal generating device which generates an alternating magnetic field at the pre-determined resonant frequency, and the signal resulting from the magnetomechanical resonance is detected by receiving equipment.
According to one embodiment disclosed in the Anderson et al. patent, the interrogation signal is turned on and off, or "pulsed", and a "ring-down" signal generated by the active element after conclusion of each interrogation signal pulse is detected.
The disclosure of the Anderson et al. patent is incorporated herein by reference.
Typically, magnetomechanical markers are deactivated by degaussing the control element, so that the bias field is removed from the active element thereby causing a substantial shift in the resonant frequency of the active element. This technique takes advantage of the fact that the resonant frequency of the active element varies according to the level of the bias field applied to the active element. Curve 20 in FIG. 1A illustrates a bias-field-dependent resonant frequency characteristic typical of certain conventional active elements used in magnetomechanical markers. The bias field level H.sub.B shown in FIG. 1A is indicative of a level of bias field typically provided by the control element when the magnetomechanical marker is in its active state. The bias field level H.sub.B is sometimes referred to as the operating point. Conventional magnetomechanical EAS markers operate with a bias field of about 6 Oe to 7 Oe.
When the control element is degaussed to deactivate the marker, the resonant frequency of the active element is substantially shifted (increased) as indicated by arrow 22. In conventional markers, a typical frequency shift upon deactivation is on the order of 1.5 kHz to 2 kHz. In addition, there is usually a substantial decrease in the amplitude of the "ring-down" signal.
U.S. Pat. No. 5,469,140, which has common inventors and a common assignee with the present application, discloses a procedure in which a strip of amorphous metal alloy is annealed in the presence of a saturating transverse magnetic field. The resulting annealed strip is suitable for use as the active element in a magnetomechanical marker and has improved ring-down characteristics which enhance performance in pulsed magnetomechanical EAS systems. The active elements produced in accordance with the '140 patent also have a hysteresis loop characteristic which tends to eliminate or reduce false alarms that might result from exposure to harmonic-type EAS systems. The disclosure of the '140 patent is incorporated herein by reference.
Referring again to curve 20 in FIG. 1A, it will be noted that the curve has a substantial slope at the operating point. As a result, if the bias field actually applied to the active element departs from the nominal operating point H.sub.B, the resonant frequency of the marker may be shifted to some extent from the nominal operating frequency, and may therefore be difficult to detect with standard detection equipment. U.S. Pat. No. 5,568,125, which is a continuation-in-part of the aforesaid '140 patent, discloses a method in which a transverse-field-annealed amorphous metal alloy strip is subjected to a further annealing step to reduce the slope of the bias-field-dependent resonant frequency characteristic curve in the region of the operating point. The disclosure of the '125 patent is incorporated herein by reference.
The techniques disclosed in the '125 patent reduce the sensitivity of the resulting magnetomechanical markers to variations in bias field without unduly diminishing the overall frequency shift which is desired to take place upon degaussing the control element. Although the teachings of the '125 patent represent an advance relative to manufacture of transverse-annealed active elements, it would be desirable to provide magnetomechanical EAS markers exhibiting still greater stability in resonant frequency.